1. Field of the Invention
The present invention generally relates to a device for testing video displays, and more particularly, to a test circuit and method for testing the performance of a display by providing a symbol on the display having an intensity which is slightly different from the background intensity, and requiring viewer to identify the symbol to verify adequate performance of the display.
2. Background of the Invention
Conventional video display devices are capable of providing high resolution, detailed images. Like other pieces of electronic equipment, however, the performance of such displays may degrade over time. The rate and degree of degradation may be such that the viewer fails to notice the overall reduction in performance. In some applications, a degradation in image quality is substantially inconsequential until the image has degraded to a point that is obviously unacceptable to the viewer. In other applications, however, this purely subjective approach to monitoring display performance is unacceptable. In military and medical applications, for example, the viewer must at all times be confident that the image content is accurate and properly displayed. Clearly, if the image is an x-ray image, the physician must be confident that variations in image content accurately reflect the target anatomy, and are not manifestations of degradations in display performance.
The conventional approach to display quality assessment involves observation of a complex reference image or test pattern. One industry standard pattern is the SMPTE pattern which was developed by a committee of the Society of Motion Picture and Television Engineers. By providing the SMPTE pattern on a display, a variety of performance parameters of the display may be assessed. As is well-known in the art, a viewer measurement device may assess parameters such as scan size, centering, geometry, resolution, aspect ratio, and brightness and contrast through observation or measurement of the displayed SMPTE pattern. The reference image approach, however, does not facilitate full display quality assessment by observation due the variability and adaptation in human vision. A variety of equipment and techniques exist which attempt to provide comprehensive analysis of electronically generated displays in a laboratory setting. None of these approaches, however, are adequately comprehensive or practical to implement as a standardized method of display assessment. These approaches are based on very technical aspects of display performance, without a relationship to viewability. While conventional approaches may permit performance comparison between different pieces of equipment, and a determination that display performance is at a certain level at a specific time, these approaches require use of expensive equipment, trained technicians, and time consuming analyses. None of these techniques are practical for use outside a laboratory environment to test the daily performance of a display device.
Assessment of the performance of a display device is complicated by the wide performance range of the human eye, its state of adaptation to the viewing environment, and the broad adjustment range available in modern displays. Modern displays anticipate variations in individual visual capability and degradations in display performance over time by providing controls for adjusting, among other things, the overall luminance level of the display (brightness) and the range of luminance between bright and dark information (contrast). Adjustment to the brightness control causes a change in luminance in all areas of the display. Brightness is adjusted to ensure that all information in the image, from black (zero luminance) to white (maximum luminance), is visible under the prevailing viewing conditions. Accordingly, the optimum setting of brightness is totally dependent upon the adaptive state of the eye and the ambient viewing conditions. Contrast is adjusted to change the luminance in all “non-black” areas. This change in luminance is proportional to the relative luminance level between white and black of the displayed information. For example, the change in luminance for mid-level gray information may be half that experienced for white information. While it is possible to establish fixed performance ranges for these controls and others, the optimum settings for these controls will vary over time, from person to person, and under the various viewing conditions.
Brightness and contrast are interactive settings which directly affect the display quality. Assuming the dynamic range of a display in terms of light intensity covers a range from 0 (black) to 300 (maximum luminance) in arbitrary units, and that when light intensity is over 255, resolution is compromised, it is desirable to adjust brightness and contrast to provide an operational range from 0 to 255. The threshold of black (the precise point at which light intensity is not discernible) is variable, however, depending upon the condition of the display device, its age, and the inherent performance capability and adaptive state of the viewer's eye. Therefore, the brightness control, used to set the black threshold point, has a range of, for example, −25 to +25, and is ideally zero. Anything less than zero is considered “blacker than black,” and is presumed to be not discernible. Once brightness is set, the contrast control is adjusted to provide a maximum light intensity in “white” areas of 255.
It is generally accepted that display of monochrome information (black and white) is the most difficult task of the display. Accordingly, a test that assesses the display's ability to present monochrome information is valuable to verify performance of the display to an acceptable level, not necessarily to provide quantitative data on what that performance is. Moreover, if the black threshold and white maximum of a display are properly set, the accuracy of the information between these limits can safely be assumed. The entire gray scale need not be analyzed, except for periodic quality assurance or for display certification. A simple test of the threshold value for the black and white extremes upon which adjustment of brightness and contrast can be made for present viewing conditions is all that is necessary. Using such a test, the brightness and contrast adjustments may be balanced to discern fine detail over the entire range of light emission from the display.
Since the invention of video displays for entertainment television with analog technology, it is presumed that there is no significant limitation in developing any level of luminance between the black and white extremes. Correspondingly, there has been little interest in the question as long as one can achieve a “pleasing” image. There has been attention to the characteristic of luminance difference obtained for a change in input signal amplitude. The characteristic of the eye is non-linear, phosphor response is non-linear, and light sensitive camera pickup devices are non-linear. There are various electronic manipulations to compensate for these characteristics, generally referred to as “gamma correction.” The goal is to provide a “linear” characteristic, electronically, for the video signal transmitting information to a display device. This has led to the acceptance of the “linear” gray scale signal whose waveform resembles a staircase as shown in FIG. 5. Traditionally the number of levels (often referred to as “steps”) has been ten, including black, white and eight intermediate “gray” luminance levels. Note that even in color systems the black to white gray scale remains the descriptor of luminance characteristics. This type of gray scale has been provided on television test charts and in various pattern generators since the birth of television. Representative is the EIA Resolution Chart, 1956, a widely used industry standard.
There have been gray scales of various increments proposed and used in various applications such as 16, 32, 64, etc. They have been presented in the familiar staircase configuration. A “ramp” signal, a gradual, linear transition from black level to white level, also has been employed to indicate the characteristic of luminance produced from a video signal input to a display device. Such signals have been developed by analog circuit techniques. However, there has been relatively little attention to the matter, as it has been assumed that the ability to develop various gray levels matches or exceeds the ability of the eye to perceive such small changes in display contrast. The most significant update in technology was brought about by the Medical Imaging Test Pattern developed by a committee of the Society of Motion Picture and Television Engineers (SMPTE), their Standard RP-133. This remains the principal tool for performance evaluation of video displays in medical imaging applications.
As video has become applied to non-entertainment uses, there has become increased interest and concern over the ability to discern information presented with very small variations in luminance, far more than is addressed by the traditional ten level gray scale. There is question especially in the region “near black” that is unaddressed by the traditional gray scale. The issue was addressed in development of the SMPTE pattern, which includes a “near black” patch at 5% above black (in terms of video signal level excursion between black and white being 100%) and a “near white” patch at 95%. These patches became the first known use of shapes that are evaluated by viewability, intended to be used as a reference for proper setting of Brightness (near black) and Contrast (near white). This level of performance test was developed in 1983, prior to the widespread use of digital technology in video.
More demanding applications are arising, notably digital mammography and advanced cardiology diagnostic devices in which performance is expected to be able to present a much finer discernible shape than previously of interest.
Through digital technology we now are able to synthesize images as well as to process video signals to a degree never before possible. However it remains necessary to convert digital signals to an analog representation to present an image for interpretation by the human eye. This is done by the Digital-to-Analog Converter (DAC). However, whereas in traditional video systems it was assumed that there could be the development of changes in luminance in an almost infinite variety, digital systems can provide waveform voltage changes, and therefore variation in luminance output from a display device, in so many discrete steps. An “8-bit” system can product 256 discrete voltage levels over the dynamic range from black to white in a video signal. A “4-bit” system can produce 16 discrete voltage levels, a “10-bit” system 1024 levels, etc. We have come to expect to be able to discern information developed in these increments. However, there is not a means to definitively evaluate such. It is assumed that the traditional presumption of infinite capability between black and white will suffice, but experience is showing that this may not be the case.
Performance of the human eye is so adaptable that it has about defied description. In confining the discussion to the ability to discern small changes in luminance we know that the eye performs in a relative manner, not in a discrete fashion. Therefore it is necessary to be able to condition, or adjust, a display device to make the desired information visible and discernible. A complication is that display devices are prone to vary in luminance production over time, so that adjustment is necessary to compensate for changes in the display device.
In the past it was widely accepted that the eye can detect and identify a “2% ” change in luminance. This is largely a meaningless number, for the actual discernability is dependent upon many conditions and is thought to vary from person to person. As digital systems become able to produce very fine but known incremental changes in luminance it is likely that more information is presented than can be discerned. This is of concern to those working with digital mammography technology and cardiologists. The degree to which this could occur is unknown and does not appear to be under formal study. Until now there has been no means to evaluate this characteristic or to check one's ability to be able to discern all that is being presented to an appropriate degree. The “5% ” reference squares in the SMPTE pattern are too coarse to be of value.
While there have been representations that a video device can produce useable “8-bit, 10-bit” video, experience teaches that this is questionable. There has not been a means provided to determine what level of performance should be expected and what information can, in fact, be discerned in an existing viewing environment. While this is inconsequential in entertainment viewing and ordinary personal computer use, it is a critical consideration in demanding applications such as medical radiology. If, in fact, the fabled “2% ” variation is valid as a threshold for ability to discern information, anything more than a 6 bit digital system (64 discrete luminance levels) is inappropriate for development of images. There is reason to believe that, with proper viewing conditions, the eye can perform better than is widely accepted.